In a lithography process for manufacturing semiconductor devices (integrated circuits), liquid crystal display devices, thin film magnetic heads or the like, conventionally full field and static exposure type projection exposure apparatuses such as a reduction projection exposure apparatus by a step-and-repeat method (a so-called stepper) have been mainly used. However, in recent years, with higher integration of semiconductor devices, scanning exposure apparatuses such as a projection exposure apparatus by a step-and-scan method (a so-called scanning stepper (also called a scanner)) have been used relatively frequently.
In the conventional stepper and scanner, design of the apparatus has been done on the assumption that deformation of a mask or a reticle (hereinafter generally referred to as a “reticle”) on which a circuit pattern is formed occurring when the reticle is sucked on a reticle holder (a platen), is similar regardless of the reticle.
However, since a resolving power close to the limitation is required in a projection optical system of this type of projection exposure apparatus, numerical aperture (NA) of the projection optical system is set to a large value in order to enhance the resolving power, and it results in a considerably shallow depth of focus (DOF). In other words, due to a narrower DOF associated with a higher NA of the projection optical system in recent years, image-forming error caused by deformation of a reticle has gradually become measurable.
That is, when a pattern surface of a reticle bends to a projection optical system side substantially evenly, the average position of an image plane also reduces, and therefore, defocus occurs when a target position of a wafer in an optical axis direction of the projection optical system is the same as that in the case where the pattern surface does not bend. Further, when the pattern surface of a reticle is deformed, a position of the pattern on the pattern surface in a direction perpendicular to the optical axis of the projection optical system also changes in some cases, so that such lateral shift of the pattern becomes a factor of distortion error. Therefore, more precise control of reticle flatness level has been required.
As the deformation of a reticle, (a) bending by self-weight, (b) deformation at the time of polishing a glass substrate itself of the reticle, (c) deformation that occurs due to the difference in flatness level between both contact surfaces of the reticle and the reticle holder (the platen) when the reticle is forcibly held on the holder by suction, and the like are considered. Since such deformation state of the reticle varies depending on each reticle, and furthermore, depending on each reticle holder of an exposure apparatus, it is necessary to measure a deformation amount of the reticle in a state where the reticle is actually held on the reticle holder of the exposure apparatus by suction in order to accurately measure the amount.
Then, to quickly measure a surface shape of a reticle, it can be considered to place a positional sensor similar to a focal point detection system (an AF sensor) by an oblique incident method for detecting the position of a wafer in the optical axis direction of the projection optical system, on a reticle stage side.
In this case, since the pattern surface of the reticle is a lower surface, that is, a surface on the projection optical system side, the positional sensor by the oblique incident method is placed in a space between the reticle stage and the projection optical system, or in the vicinity of the space. Particularly in the scanning exposure apparatus, the reticle stage needs to maintain sufficient rigidity to prevent it from being deformed even when stress is applied when accelerating and decelerating for the purpose of synchronous scanning. Therefore, in many cases, the reticle stage takes a constitution having sufficient thickness to the limit where it almost contacts the projection optical system. Furthermore, since the designing of the projection optical system is easier when the space between the reticle and the projection optical system is narrower, the space between the reticle and the projection optical system tends to be even narrower with higher accuracy of the projection optical system. Accordingly, it has become difficult to place a positional sensor for reticle between the projection optical system and the reticle.
In view of such points, even in the case the space between the stage on the reticle side and the projection optical system is narrow and it is difficult to install a sensor for measuring the shape of the pattern surface of the reticle in the space, a scanning exposure method and a scanning exposure apparatus that can measure the shape of the pattern surface and obtains good image-forming characteristics have been suggested (refer to Patent Document 1, 2, 3 and the like).
However, in the scanning exposure method and the scanning exposure apparatus according to Patent Documents 1 to 3, a positional sensor on the reticle side (a reticle AF sensor) is essential, and the space for placing the AF sensor is necessary in the vicinity of the projection optical system although it may not be directly above the projection optical system, which results in an insufficient degree of freedom in design of the projection optical system and the reticle stage.
[Patent Document 1] Kokai (Japanese Unexamined Patent Application Publication) No. 11-045846 publication
[Patent Document 2] Kokai (Japanese Unexamined Patent Application Publication) No. 11-026345
[Patent Document 3] U.S. Pat. No. 6,549,271